Journal
SOIL BIOLOGY & BIOCHEMISTRY
Volume 150, Issue -, Pages -Publisher
PERGAMON-ELSEVIER SCIENCE LTD
DOI: 10.1016/j.soilbio.2020.108012
Keywords
Birch effect; Isotope; Mass spectrometry; Osmolyte; Pool dilution; Trehalose
Categories
Funding
- University of Sydney [G199770]
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When dry soils are re-wet there is a pulse of soil respiration that could be partially fuelled by the osmolytes that had accumulated within microbial biomass during the preceding drying phase. We hypothesize that if the pulse of respiration in re-wet soil is because microbes respond to the hypo-osmotic shock of re-wetting by expelling osmolytes and then re-capture osmolytes for use as respiratory substrates: a) rewetting will lead to a massive flux of osmolytes from microbial biomass into the extracellular fraction of soil, and b) following rewetting there will be a decrease in the total pool of osmolytes (i.e. in microbial biomass + the extracellular fraction of soil) that is proportional to the extra C that is respired by rewet soil compared to permanently moist soil. We used mass spectrometry and isotope pool dilution to estimate intracellular and extracellular pools, and extracellular fluxes, of seven putative osmolytes (trehalose, betaine, carnitine, hercynine, acetyl-carnitine, choline, proline) and the protein amino acid alanine in a soil that was permanently moist and a soil that had been slowly dehydrated and was then re-wet to the same water content as the permanently moist soil. In moist soil, the influx and efflux of trehalose through the extracellular pool was around 50% faster than of alanine and proline, and all three compounds had mean residence times in the extracellular solution of less than 10 min. Quaternary ammonium compounds (betaine, carnitine, hercynine, acetyl-carnitine, choline) had gross fluxes 5 to 33 times slower than for trehalose, but despite these slower fluxes mean residence times were 45 min or less. Microbial biomass of dry soil had a three times larger pool of osmolyte-C than moist soil, while the extracellular fraction of dry soil had a six times larger pool of osmolyte-C than the moist soil. Microbial biomass of dry soil was 2.5-4 times smaller than for permanently moist soil, and thus per unit microbial biomass the amount of osmolyte-C in dry soil was 7.5 to 12 times greater than the moist soil. The contribution of intracellular osmolytes to solute potential was likely too small to maintain turgor in the dry soil, and thus it is possible the large extracellular and intracellular accumulation of osmolytes was functioning to stabilise membranes and foster desiccation tolerance. The fluxes of osmolytes into and out of solution resumed rapidly after re-wetting. There was no evidence that rewetting led to an increased flux of osmolytes from microbial to extracellular pools, as would be expected if re-wetting caused microbes to expel osmolytes or be lysed. Extracellular pools of most osmolytes changed little following re-wetting, whereas the extracellular pool of trehalose decreased significantly. Isotope pool dilution revealed that the constant extracellular concentrations of most osmolytes following rewetting was because rates of removal from solution balanced the rates of entry, while the decreasing extracellular pool of trehalose reflected that it was removed from solution almost three times faster than it entered solution. The decreasing extracellular and intracellular pools of osmolytes, especially trehalose, following rewetting could theoretically account for the faster respiration of re-wet soil than permanently moist soil for several hours. However, respiration of re-wet soil was faster than permanently moist soil for five days following re-wetting and over this longer time scale the respiration pulse must be fuelled by sources of C in addition to osmolytes.
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